Question

How does the atomic number affect the atomic radius?

Answer 1

The atomic number tells the number of electrons in the atom and atomic radius is the distance between the center of the nuclei to the last or outermost electron in the atom. While moving from left to right in the period, the atomic radius is inversely proportional to atomic number, and while moving down in the group, the atomic radius is directly proportional to atomic number.

Complete answer:
The atomic number is the number of electrons and the number of protons present in the atom. Protons and neutrons are within the nucleus but the electrons are outside the nucleus and they are attracted by the nucleus with a nuclear charge.
Atomic radius is the distance between the center of the nuclei to the last or outermost electron in the atom.
While moving from left to right in the period, the atomic number increases so, the number of electrons also increases and this increase is with the same shell, therefore due to the increase in the number of electrons within the same shell the nuclear charge will increase and the electrons will come closer and the radius will decrease. So, while moving from left to right in the period, the atomic radius is inversely proportional to the atomic number.
But when we move in a group i.e., up to down, the number of electrons increases but the outermost number remains the same, the number shell increases. As the number of shells increases, the distance increases, and the atomic radius increases. So, while moving down in the group, the atomic radius is directly proportional to the atomic number.

Note:
With the help of atomic radius, we can also decide many other factors like electronegativity, ionization enthalpy, electron gain enthalpy, etc, the trend of all these factors is reverse of the order of atomic radius.